Photo-assisted oxidation of inorganic species in aqueous solutions

ABSTRACT

A method for oxidizing an inorganic species in an aqueous solution comprises the steps of: (i) supplying an oxidizable source of sulphur, and oxygen to the solution; and (ii) irradiating the solution with UV light such that both the inorganic and sulphur species are oxidized.

FIELD OF THE INVENTION

The present invention relates to a method for oxidising inorganicspecies in aqueous solutions, and more particularly, to the treating ofcontaminants in e.g. human drinking water, and industrial waste watersand process liquors. However, it should be appreciated that theinvention can be employed wherever it is necessary to oxidise aninorganic species in aqueous solutions for whatever reason.

BACKGROUND TO THE INVENTION

Dissolved sulphur dioxide or sulphite is usually considered to be areducing agent. Further, it is known that the oxidation of sulphite isaccelerated through exposure to UV radiation (Matthews, J. H. et.al. J.Am. Chem. Soc. 1917,39, 635). Matthews teaches, however, that oxidationis retarded by the presence of trace amounts of various species. Inaddition, no change in the oxidation state of these species wasobserved.

Many drinking water supplies across the world are contaminated by tracecontaminants including arsenic, iron and manganese. World HealthOrganisation standards require very low levels of contaminants (forexample arsenic-a 10 ppb limit). The presence of manganese gives rise to“dirty water” problems and can result in soiling of clothes and stainingof household fixtures when present in concentrations in excess of 20 ppbin drinking water.

Many waste waters and mineral processing liquors from industry alsoinclude arsenic, iron, manganese and cerium, and in the field of nucleartechnology, uranium.

As part of the removal process, chemical oxidants such as chlorine,ozone and permanganate are often used. However, these oxidants can giverise to harmful byproducts such as chloroform, and the presence ofresidual permanganate can produce discoloured waters.

SUMMARY OF THE INVENTION

The present invention provides a method for oxidising an inorganicspecies in an aqueous solution comprising the steps of:

-   -   (i) supplying an oxidisable source of sulphur, and oxygen to the        solution; and    -   (ii) irradiating the solution with UV light such that the        species is oxidised.

In the present invention, oxygen is advantageously used as the oxidisingagent, with no residual contaminating after-effects. Sulphur sources canbe selected, (e.g. sulphite) such that in the oxidising procedure, arelatively benign product is produced (e.g. sulphate). Although thefinal product of using sulphite is a relatively benign dissolvedsulphate, it is preferable to use it sparingly especially if anion-exchange process is subsequently used to remove the contaminant(e.g. arsenic). In this case dissolved sulphate of no more than 25 mg/Lis preferred in order to obtain effective arsenic(V) removal (sulphateand arsenate compete for sites on the ion-exchange material).

The oxidisable sources of sulphur can be SO₃ ²⁻, S₂0₃ ²⁻, S₄0₆ ²⁻,SO₂(g), aqueous SO₂, or HSO₃ ⁻. However, sulphur dioxide and sulphiteare preferred sources.

Typically the process is applied in the treatment of trace quantities ofinorganic species but the process can also find application with moreconcentrated quantities of contaminants.

Typically the species oxidised includes one or more of arsenic,manganese, cerium and/or iron.

Typically the ultraviolet light employed has a wavelength of about 254nm. Radiation can be supplied continuously or in pulses. Furthermore,low, medium or high pressure mercury arc lamps can be used as the sourceof the UV radiation. It was also noted that UV wavelengths of 254 nmfrom a lamp source advantageously disinfected water so treated.

Typically the oxygen is sparged into the aqueous solution as air butother methods of addition are possible. As indicated above, the solutionis typically a drinking water solution, an industrial waste water orprocess liquor etc.

Typically the pH of the solution is, if necessary, adjusted to beapproximately neutral or basic.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thepresent invention, preferred forms of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings and the following non-limiting examples. In the drawings:

FIG. 1 is a graph that plots the increase in arsenic(V)-concentrationand concomitant decrease in sulfite-concentration as a function ofillumination time using a 15 W low-pressure mercury lamp. Thecorresponding change in arsenic(V) concentration in darkness is alsoshown. (Initial conditions: 1.7 liter of solution containing 470 ppbarsenic(III) in the presence of 10 mg/L sulfite, solution pH adjusted to9 using sodium carbonate).

FIG. 2 is a graph that plots the increase in arsenic(V) concentrationsas a function of illumination time using a 15 W 254 nm lamp. (Initialconditions: 1.7 liter of solution containing arsenic(III) concentrationof approximately 470 ppb, pH adjusted to 9 using sodium carbonate,initial sulfite concentrations varied from 0 to 12 mg/L).

FIG. 3 is a graph that plots arsenic(V) concentrations as a function ofelapsed time when solutions (1.7L) containing arsenic(III) at aconcentration of 470 ppb, at various controlled pHs, were illuminatedwith a 15 W 254 nm lamp. Sodium sulphite solution was added at a doserate of 2 mg/L/min and air was sparged at a rate of 2.5 L/min. Data withno UV illumination (dark) are also shown.

FIG. 4 is a graph that plots arsenic(V) concentrations as a function ofelapsed time when a solution (1.7L) containing arsenic(III) at aconcentration of about 20 mg/L, at pH 6.5, was illuminated with a 15 W254 nm lamp. Sulphur dioxide gas was injected at a rate of about 0.02L/min and air was sparged at a rate of 2.5 L/min. Data with no UVillumination (dark) are also shown.

FIG. 5 is a graph that plots arsenic(V) concentrations as a function ofelapsed time when solutions (1.7L) containing arsenic(III) concentrationof 470 ppb, at pH 6.5, were illuminated with a 15 W 254 nm lamp. Sodiumthio-sulphate solution was added at various dose rates (in mg/L/min) andair was sparged at a rate of 2.5 L/min. Data with no UV illumination(dark) are also shown.

FIG. 6 is a graph that plots arsenic(V) concentrations as a function ofelapsed time when a solution (1.7L) containing arsenic(III) at aconcentration of 470 ppb, at pH 6.5, was illuminated with a 15 W 254 nmlamp. Sodium tetra-thionate solution was added at a dose rate of 2mg/L/min and air was sparged at a rate of 2.5 L/min. Data with no UVillumination (dark) are also shown.

FIG. 7 is a graph that plots residual manganese concentrations as afunction of elapsed time when solutions (1.7L) containing manganese(II)concentration of about 500 ppb, at pH 8.5, were illuminated with a 15 W254 nm lamp. Sodium sulphite solution was added at a dose rate of 2mg/L/min and air was sparged at a rate of 2.5 L/min. The oxidisedmanganese was removed using a 0.025 micron membrane filter. Tofacilitate manganese removal, ferric chloride (6.2 mg Fe/L) was added in2 of the 4 tests. Data with no UV illumination (dark) are also shown.

FIG. 8 is a graph that plots residual manganese concentrations as afunction of elapsed time when a solution (1.7L) containing manganese(II)at a concentration of about 20 mg/L, at pH 9.5, was illuminated with a15 W 254 nm lamp. Sodium sulphite solution was added at a dose rate of80 mg/L/min and air was sparged at a rate of 2.5 L/min. Ferric chloridewas added at 6.2 mg Fe/L to facilitate the manganese removal. Data withno UV illumination (dark) are also shown.

FIG. 9 is a graph that plots iron(II) concentrations as a function ofelapsed time when a solution (1.7L) containing iron(II) at aconcentration of about 20 mg/L, at pH 2, was illuminated with a 15 W 254nm lamp. Sodium sulphite solution was added at a dose rate of 20mg/L/min, air was sparged at a rate of 2.5 L/min. Data with UV withoutsulphite, and no UV illumination (dark) are also shown.

FIG. 10 is a graph that plots cerium(IV) concentrations as a function ofelapsed time when a solution (1.7L) containing cerium(III) at aconcentration of 20 mg/L, at pH 6.5, was illuminated with a 15 W 254 nmlamp. Sodium sulphite solution was added at a dose rate of 20 mg/L/min,air was sparged at a rate of 2.5 L/min. Data with UV illumination butwithout sulphite, and no UV illumination (dark) are also shown.

MODES FOR CARRYING OUT THE INVENTION

Preferred forms of the present invention find application in thetreatment of drinking water, waste waters and mineral processingliquors. It should be appreciated, however, that the invention hasbroader applications.

With drinking water treatment, it is desirable to remove traceoxidisable contaminants, such as arsenic and manganese. In at leastpreferred forms, contaminants are oxidised and then removed underneutral or slightly alkaline conditions.

In the treatment of waste waters and mineral processing liquors, it isdesirable to neutralise and/or remove (depending on the final use of thewater or liquor) species such as arsenic, iron and manganese. In theseapplications, however, oxidation may take place in acid, neutral oralkaline conditions.

Manganese-related “dirty water” problems are a significant water qualityissue to water supply authorities. It is understood that 40 percent ofpublic water supplies in the United States have manganese concentrationsexceeding levels of 10-20 ppb.

Manganese is also a problem in processed wastes in the milling ofuranium ores and in acid mine drainage. Manganese is often present inthe ore to be milled, and may be also introduced as an oxidant in theform of a pyrolusite (MnO₂) which is an oxidant used in the leaching ofuranium.

In industrial process liquors, it is necessary to oxidise various metalions as part of the overall processing in the plant.

Details of various preferred process operating parameters are nowdescribed.

Source of Radiant Energy

Any source of radiant energy in the UV region of the electromagneticspectrum was observed to be useful, provided that the radiation wasabsorbed by the dissolved sulphur compound which was acting as thephoto-initiator of the process. Low pressure mercury arc lamps were usedfor the oxidation of dissolved arsenic(III), manganese(II), iron(II) andcerium(III). Typical UV wavelengths of less than 300 nm were employed(preferably about 254 nm).

Choice of Photo-Absorber

Dissolved sulphur species absorbed the supplied UV light and wereoxidised by dissolved oxygen. These sulphur species were used up(oxidised) during the photochemical reaction. Dissolved sulphur(IV)species derived from the addition of sodium sulphite included SO₂ ³⁻,HSO₃ ⁻ or H₂SO₃ depending on the pH value of the solution. The samedissolved sulphur species were obtained by dissolving SO₂ gas in waterwhich gives aqueous SO₂ which, in turn, converted to sulphurous acid(H₂SO₃). Sulphorous acid dissociated to HSO₃ ⁻ and SO₃ ²⁻ at higher pHconditions. Dissolved sulphite from sodium sulphite was used for theoxidation of manganese(II).

Other partially oxidised sulphur species (sulphur(VI) as in sulphatecompounds having fully oxidised sulphur species) obtained from thedissolution of sodium thiosulphate or sodium tetrathionate were alsoused as the photo-absorber.

Furthermore, dissolved sulphite was obtained by sparging sulphur dioxidegas or a gas mixture of sulphur dioxide and air/oxygen/nitrogen into thesolution. Thus, the forms of sulphur employable included SO₃ ²⁻,SO_(2(g)), aqueous SO₂, HSO₃ ⁻, S₂O₃ ²⁻ and S₄O₆ ²⁻.

Source of Oxidant

Oxygen was the oxidant for the photochemical oxidation process. It wastypically supplied at about 0.2 atmospheres partial pressure by aeratingthe reaction mixture. Alternatively, oxygen was supplied by sparging agas mixture of sulphur dioxide with air, or an oxygen/nitrogen mixtureinto the solution (or any other compatible gas source). Oxygen partialpressures greater than or less than 0.2 atm can also be used asappropriate.

Illumination was achieved by placing a lamp within a quartz envelopeinside the reaction vessel (alternatively, the light can be directedfrom above the solution). Types of lamp used included a high or lowpressure mercury arc lamp or a xenon arc lamp.

It was noted that where the UV source chosen emitted light at awavelength at about or below 190 nm, ozone was generated from thedissolved oxygen; (ozone is a powerful oxidant which can oxidisearsenic(III) and manganese(II)). For the examples described below,non-ozone producing lamps were employed.

EXAMPLES

Non-limiting examples will now be described.

Photo-Oxidation of Dissolved Arsenic(III)

A reaction mixture (1700 mL) containing 470 μg/L As(III) (typicalconcentrations in ground water in areas where arsenic is leached fromthe naturally occurring arsenic-containing minerals) and 10 mg/L ofdissolved sulphite (SO₃ ²⁻) was prepared as follows: the sulphite stocksolution was prepared by dissolving sodium sulphite salt indemineralised water; the arsenious acid (As(III)) solution was obtainedby dissolving arsenic trioxide in warm, demineralised water. The pH ofthe reaction mixture was adjusted to 9 with the addition of sodiumcarbonate (because groundwaters typically have significant carbonatealkalinity). The solution was then aerated by the injection of finebubbles of air.

In the absence of UV illumination, no significant oxidation of As(III)was observed (FIG. 1). When a 15 W low pressure mercury lamp wasswitched on to illuminate the reaction mixture, the oxidation of As(III)and S(IV) proceeded rapidly (FIG. 1).

The experiments were repeated using various initial concentrations ofdissolved sulphite, namely from 0 to 12 mg/L of dissolved sulphite. Asshown in FIG. 2, the rate of As(III) oxidation was strongly dependent onthe initial sulphite concentration when it was less than 8 mg/L. FIGS. 1and 2 demonstrate that both UV light and dissolved sulphite were neededfor the photo-oxidation reaction to occur.

FIG. 3 shows that the arsenic oxidation rate was increased by theincrease in the solution pH. During these test runs, the solution pH wascontrolled at the selected value using an automatic titrator which addedsodium hydroxide solution when required. Sodium sulphite was added bythe continuous injection of a stock solution (17 g/L of sulphite) at aprecisely controlled flow rate using a titrator in order to give a doserate of 2 mg/L i.e. 0.2 mL/min of the stock solution was injected intothe 1.7 L of reaction mixture. This method of sulphite dosing is moreefficient than the procedure described for FIGS. 1 and 2 where thesodium sulphite was added in a single dose. It also simulates theprocedure where SO₂ gas is used. Air was sparged at a rate of 2.5 L/min.

Sulphur dioxide gas was used instead of sodium sulphite as shown in FIG.4. Arsenic(III) was oxidised when sulphur dioxide and air was bubbled inthe absence of UV illumination (auto oxidation process). However, theoxidation rate was accelerated when the reaction mixture wasilluminated. It was observed that significant concentrations ofdissolved sulphite were present in the reaction mixture indicating thatan excess sulphur dioxide was sparged. Hence, the difference between theresults of the ‘light’ and ‘dark’ experiments was not large as could beachieved.

Sodium thiosulphate can be substituted for sodium sulphite as shown inFIG. 5. Similarly, sodium tetrathionate was used as the photo-absorberas demonstrated in FIG. 6.

Actinometry determination using potassium ferrioxalate showed that amaximum of 6 Watts of 254 nm radiation produced by the 15 W lamp wasabsorbed by the reaction mixture. Total As and As(III) concentrationswere determined using atomic absorption spectroscopy with hydridegeneration. Concentrations of As(V) in the reaction mixture weredetermined using the molybdenum blue spectrophotometric method (JohnsonD. and Pilson M., Analytical Chimica Acta, 58, 289-299 (1972)). Sulphiteconcentrations were also determined spectrophotometrically (Humphrey R.E., Ward M. H. and Hinze W., Analytical Chemistry, 42, 698-702 (1970)).

Photo-Oxidation of Dissolved Manganese(II)

A reaction mixture (1700 mL) containing 500 μg/L Mn(II) (typicalconcentrations in surface and ground water are less than 1 mg/L) and 10mg/L SO₃ ²⁻, was prepared as follows: the sulphite stock solution wasprepared by dissolving sodium sulphite salt in demineralised water; theMN(II) stock solution was obtained by dissolving MnSO₄.4H₂O indemineralised water. The pH of the reaction mixture was 6.5 and it wasaerated by the injection of fine bubbles of air.

After a 15 W low pressure mercury lamp was switched on to illuminate thereaction mixture for 2 minutes, the reaction mixture became cloudybecause of the appearance of grey/black suspended particles indicatingthat an oxide of manganese had been formed. A 25 mL sample was collectedand its pH (4-5) was adjusted to 7 using dilute sodium hydroxidesolution in order to coagulate colloidal manganese oxide particles.After 30 minutes, to allow the precipitated grey/black particlessufficient time to coagulate, the sample was filtered using an Amiconunit fitted with 0.025 μm membrane. The dissolved Mn concentration inthe filtrate was 22 μg/L. This indicates that most of the dissolvedMn(II) was oxidised to Mn(III)/Mn(IV) and precipitated as manganeseoxide (which is black).

When the same procedures were repeated without illumination, thereaction mixture remained clear and colourless. A sample was taken after30 minutes and subjected to the coagulation and filtration procedure.The manganese concentration in the filtrate was 505 ppb (Table 1).

Dissolved manganese concentrations were analysed using ICP-MS, ICP-AESor atomic absorption spectroscopy with a graphite furnace.

The results of one SUCH procedure are summarised in Table 1.

TABLE 1 Residual manganese concentration in water after filtration.Concentration in parts per billion Initial Concentration 511 ppb After 2min illumination at pH 6.5 in  22 ppb the presence of 10 mg/L sulfiteAfter 30 minutes without illumination 505 ppbPhoto-Oxidation of Other Dissolved Compounds

The above procedures can also be employed with the photo-oxidation ofdissolved compounds such as Se(IV), CN⁻, Fe(II), Ni(II), V(IV), U(IV),and Ce(III). Oxidation can be demonstrated by making up a reactionmixture containing an appropriate concentration of one or more of thesecompounds. Dissolved sulphur species can be obtained from stock solutionwhich can be prepared by dissolving either the equivalent sodium orcalcium salt in water. The mixture can then be divided into threeportions for a set of three tests:

-   1. No UV illumination. To the first portion of the reaction mixture,    an appropriate amount of sulphite or sulphide is added from a    concentrated stock solution. The mixture is aerated with a    nitrogen-oxygen mixture of known oxygen partial pressure. Speciation    of the oxidation state of the target substance and the sulphur at    several time intervals was used to determine the reaction rate.-   2. The second portion of the reaction mixture is aerated with the    same oxygen/nitrogen gas mixture used in Test 1 and illuminated    without the addition of a sulphur compound.-   3. The final portion is aerated with the same addition of sulphite    or sulphide as used in Test 1 and the light was then turned on to    start the experiment. The oxidation rate is determined as above for    As(III) and Mn(II).

Illumination can be achieved by placing a lamp either within an envelopeinside the reaction vessel or such that the light radiates from above.Types of lamps used can include a high or low pressure mercury arc lamp,a Xenon arc lamp or a blacklight blue fluorescent tube.

It would be observed that the rate of oxidation of the target substancein Test 3 is greater than that in either Tests 1 or 2.

Procedure for Experiments Using Sulphur Dioxide Gas

The photo-oxidation reaction can proceed just as well when sulphurdioxide gas is used instead of sulphite salt. In order to demonstratethis, for each target substance, a set of three tests can be performedas in the previous section. The reaction mixture can be sparged with afine stream of gas bubbles. The partial pressure of oxygen, sulphurdioxide and nitrogen can be independently varied in the gas stream from0 to 100%.

-   Test A Sulphur dioxide is added to the oxygen/nitrogen gas stream at    a known partial pressure, in the absence of any illumination, and    the rate of oxidation was determined by speciating the oxidation    state of the target substance after several time intervals.-   Test B The vessel is designed so that the second portion of the    reaction mixture was illuminated with light from a lamp, in the    absence of sulphur dioxide. The slow background oxidation rate (if    any) is determined by speciating the target substance for oxidation    state at several time intervals.-   Test C The third portion of the reaction mixture is placed in the    reaction vessel. Sulphur dioxide was added to the gas stream and the    lamp is switched on at the same time, to mark the beginning of the    experiment. The sulphur dioxide partial pressure is the same as that    in Test A and the illumination source and lamp intensity were the    same as that in Test B.

The rate of oxidation of the target substance in Test C would be greaterthan that in Tests A or B.

The pH of the reaction mixture and the addition of sodium sulphitesolution were controlled using automatic titrators as described above.The oxidation of manganese was evidenced by the appearance of grey/blacksuspended particles indicating that an insoluble oxide of manganese(III)or (IV) had been formed. Preliminary measurements using electronparamagnetic resonance spectroscopy confirmed that the concentration ofdissolved Mn(II) decreased with the elapsed time of illumination.

The precipitated manganese particles were removed using an Amicon unitfitted with 0.025 micron membrane filter. As shown in FIG. 7, theaddition of ferric chloride solution to give a concentration of about 6mg Fe/L in the reaction mixture improved the removal of manganese fromsolution. The residual manganese concentrations were analysed usingICP-MS, ICP-AES or atomic absorption spectroscopy with graphite furnace.

As shown in FIG. 7, at pH 8.5, the rate of removal of manganese fromsolution was accelerated by the illumination of the reaction mixtureusing UV light from a low-pressure mercury lamp. The oxidation of a moreconcentrated solution of Mn(II) at pH 9.5 is depicted in FIG. 8. Here,the dosing rate of sulphite was increased to 80 mg/L/min to account forthe initial Mn(II) concentration of 20 mg/L.

Photo-Oxidation of Dissolved Iron(II) in Acid Conditions

The oxidation of iron(II) was followed by periodically measuring theresidual dissolved iron(II) concentration in the reaction mixture. Thiswas determined spectrophotometrically using ferrozine reagent (Stookey,Analytical Chemistry, Vol. 42, No. 7, 1970).

FIG. 9 shows iron(II) concentrations as a function of elapsed time whena solution (1.7L) containing iron(II) at a concentration of about 20mg/L, at pH2, was illuminated with a 15 W 254 nm lamp. Sodium sulphitesolution was added at a dose rate of 20 mg/L/min, air was sparged at arate of 2.5 L/min. The oxidation data with UV illumination but withoutsulphite indicated that the oxidation of Fe(II) by dissolved oxygen wasaccelerated by UV illumination. This was due to the fact that dissolvedFe(II), which is a mild photo-absorber of light at 254 nm wavelength,photo-initiated and sustained the oxidation reaction. Dissolved Fe(II)was oxidised in the presence of dissolved sulphite and oxygen without UVillumination (dark) (known as the auto-oxidation reaction).

Photo-Oxidation of Cerium(III)

The oxidation of cerium(III) was followed by measuring the concentrationof cerium(IV) in solution using a volumetric titration method (Vogel A.I. ‘A text-book of quantitative inorganic analysis’ third edition,Longmans 1961, page 318). Data from the three test runs, with UVillumination and sulphite dosing, with UV illumination but withoutsulphite dosing, and without UV illumination but with sulphite dosingare given in FIG. 10.

As shown in FIG. 10, unlike the case for iron(II), the auto-oxidationreaction (in the dark) was not sufficient to oxidise dissolvedcerium(III). However, like iron(II), dissolved cerium(III) absorbed UVlight at 254 nm and photo-initiated the oxidation reaction. Thephoto-oxidation reaction was clearly accelerated by the addition of 20mg/L of sulphite per minute.

Whilst the invention has been described with reference to a number ofpreferred embodiments it should be appreciated that the invention can beembodied in many other forms.

1. A method for oxidizing selenium, vanadium, nickle, arsenic,manganese, cerium or uranium in an aqueous solution comprising the stepsof: (i) supplying an oxidizable source of sulphur as a photoabsorber,and oxygen to the solution; and (ii) irradiating the solution with UVlight such that the selenium, vanadium, nickle, arsenic, manganese,cerium or uranium is oxidized.
 2. The method as claimed in claim 1,wherein the oxidizable source of sulphur is chosen from the groupconsisting of one or more of SO₃ ², SO₂(g), aqueous SiO₂, HSO₃ ⁻, S₂O₃²⁻ and S₄O₆ ²⁻.
 3. The method as claimed in claim 1, wherein theinorganic species is present in the aqueous solution in tracequantities.
 4. The method as claimed in claim 1, wherein the wavelengthof UV light is less than 300 nm.
 5. The method as claimed in claim 1,wherein the oxygen supplied to the solution is derived from air.
 6. Themethod as claimed in claim 1, wherein the oxygen supplied to thesolution has a partial pressure of about 0.2 atmospheres.
 7. The methodas claimed in claim 1, wherein the aqueous solution is one of: drinkingwater, industrial waste water, or an industrial process liquor.
 8. Amethod of oxidizing at least one of selenium, vanadium, nickle, arsenic,manganese, cerium and uranium in an aqueous solution, the methodcomprising the steps of: (i) supplying an oxidizable source of sulphuras a photoabsorber to the solution; (ii) supplying oxygen to thesolution; and (iii) irradiating the solution with UV light such thatoxidation of at least one of the selenium, vanadium, nickle, arsenic,manganese, cerium and uranium occurs.